Thermoelectric compositions and process

ABSTRACT

A process for producing bulk thermoelectric compositions containing nanoscale inclusions is described. The thermoelectric compositions have a higher figure of merit (ZT) than without the inclusions. The compositions are useful for power generation and in heat pumps for instance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/445,662 filed Jun. 2, 2006 and incorporated in its entirety byreference herein, which claims the benefit of U.S. Provisional PatentApplication No. 60/687,769 filed Jun. 6, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for producing novel bulkthermoelectric compositions with nanoscale inclusions which enhance thefigure of merit (ZT). In particular, the present invention relates tothermoelectric compositions wherein the nanoscale inclusions are visibleby conventional nanoscale imaging techniques such as transmissionelectron microscopy (TEM) imaging. They are useful for power generationand heat pumps.

2. Description of the Related Art

The prior art in thermoelectric materials and devices is generallydescribed in U.S. Pat. No. 5,448,109 to Cauchy, U.S. Pat. No. 6,312,617to Kanatzidis et al., as well as published application 2004/0200519 A1to Sterzel et al. and 2005/0076944 A1 to Kanatzidis et al. Each of thesereferences is concerned with increasing the figure of merit (ZT) whichis directly influenced by the product of electrical conductivity and thesquare of the thermopower divided by the thermal conductivity. Generallyas the electrical conductivity of a thermoelectric material isincreased, the thermal conductivity is increased. The efficiency of thethermoelectric device is less than theoretical and may not besufficiently efficient for commercial purposes.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide relativelyefficient bulk thermoelectric materials. Further, it is an object of thepresent invention to provide a process for the preparation of thesethermoelectric materials. Further, it is an object of the presentinvention to provide thermoelectric materials which are relativelyeconomical to prepare compared to artificial deposited superlattice thinfilm thermoelectric materials. These and other objects will becomeincreasingly apparent by reference to the following description and thedrawings.

The present invention relates to a thermoelectric composition whichcomprises: a homogenous solid solution or compound of a firstchalcogenide providing a matrix with nanoscale inclusions of a secondphase which has a different composition wherein a figure of merit (ZT)of the composition is greater than that without the inclusions.Preferably the inclusion has been formed by spinodal decomposition as aresult of annealing the composition at an appropriate temperature lessthan a melting point of the homogenous solid solution based upon a phasediagram. Preferably the inclusion has been formed by matrixencapsulation as a result of doping of a mol ten solution of the matrix.Preferably the inclusion has been formed by nucleation and growth of theinclusion by cooling a molten solution of the matrix.

The present invention also relates to a thermoelectric composition whichcomprises a homogenous solid solution or compound of a chalcogenidecomprising a uniform precipitated dispersion of nano particles of atleast two different metal chalcogenides wherein the chalcogen isselected from the group consisting of tellurium, sulfur and selenium.Preferably the composition has been formed by spinodal decomposition ofthe solid solution.

The present invention also relates to a thermoelectric composition whichcomprises a homogenous solid solution or compound of a chalcogenide withdispersed nano particles derived from a metal or a semiconductor whichhave been added to the chalcogenide.

The present invention also relates to a thermoelectric composition whichcomprises a homogenous solid solution or compound of a chalcogenidewhich has been annealed at a temperature which allows the formation ofnano particles having a different composition than the solid solution orcompound.

Further, the present invention relates to a composition wherein theinclusion has been formed by matrix encapsulation as a result of dopingof a molten solution of the matrix.

Still further, the present invention relates to a composition whereinthe inclusion has been formed by nucleation and growth of the inclusionby cooling a molten solution of the matrix.

The present invention further relates to a process for preparing athermoelectric composition which comprises:

(a) forming a liquid solution or compound of a first chalcogenide and asecond phase which has a different composition;

(b) cooling the solution rapidly so that a solid solution of the firstchalcogenide as a matrix and the second phase as a nanoscale inclusionis formed, so that the figure of merit is greater than without theinclusions. Preferably the inclusion is formed by spinodal decompositionas a result of annealing the composition at an appropriate temperatureless than a melting point of the homogenous solid solution based upon aphase diagram. Preferably the inclusion is formed by matrixencapsulation as a result of cooling a molten solution of the matrix.Preferably the inclusion is formed by nucleation and growth of theinclusion in a supersaturated solid solution of the matrix. Preferablythe chalcogenides are of a chalcogen selected from the group consistingof tellurium, sulfur and selenium. Preferably the inclusions are betweenabout 1 and 200 nanometers.

The substance and advantages of the present invention will becomeincreasingly apparent by reference to the following drawings and thedescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the lowest thermal conductivity exhibited bythe PbTe—PbS 16% nanocomposite.

FIG. 2A is a theoretical phase diagram for A and B.

FIG. 2B is a diagram showing the spatial difference in the compositionof the phases.

FIG. 3A shows a PbTe—PbS phase diagram. FIG. 3B is a PbTe—PbS x %nano-composite reaction and post-annealing profile taking advantage ofspinodal decomposition. FIGS. 3C and 3D are high resolution TEM imagesof a PbTe—PbS 16% spinodally decomposed system.

FIG. 4A is a schematic of the matrix encapsulation. FIG. 4B is a graphof a PbTe—Sb heating profile. FIG. 4C is a PbTe—Sb (2%) Bright FieldImage. FIGS. 4D and 4E are corresponding Bright and Dark Field TEMimages of PbTe—InSb (2%). FIGS. 4F to 4K are transmission electronmicrographs showing dispersed nanoparticles of Sb within a crystallinematrix of PbTe. Similar size, shape, and volume fraction are observedfor (A) PbTe—Sb (2%) (B) PbTe—Sb (4%) (C) PbTe—Sb (8%) and (D) PbTe—Sb(16%). Because the 8 and 16% samples contain distinct Sb regions theimages shown in FIGS. 4C and 4D are from the PbTe rich region. FIG. 4Eis a high resolution transmission electron micrograph showing severalnanoprecipitates of Sb coherently embedded within the matrix of PbTe.Embedded particles help to maintain high electron mobility while servingas a site for phonon scattering to reduce the thermal conductivity. FIG.4F is a high resolution micrograph of the PbTe—Bi (4%) system alsoshowing embedded particles in the PbTe matrix. FIG. 4L is a graphshowing thermal conductivity as a function of temperature.

FIGS. 5A and 5B are scanning electron micrographs of PbTe+Pb (2%)+Sb(3%). Large regions or ribbons, several hundred microns in length,composed of a Pb—Sb eutectic appear throughout the sample. Similarmicrostructure is observed for other samples with similar composition.

FIGS. 6A and 6B show powder x-ray diffraction clearly indicatingadditional phases of Pb and Sb as revealed by the magnified insetbetween 25 and 40 degrees. The peak at ˜29 deg. corresponds to elementalSb while the peaks at 31 and 36 deg. can be indexed according toelemental Pb.

FIGS. 7A and 7B show (7A) low magnification transmission electronmicrographs showing dispersed particles of Pb and Sb within the PbTematrix. FIG. 7B shows high magnification TEM micrographs showing theparticles appear coherently embedded in the matrix.

FIG. 8 is a graph showing lattice thermal conductivity as a function ofPb/Sb ratio at 350K and 600K. A strong linear dependence of the latticethermal conductivity is observed as the ratio is varied.

FIG. 9A is a schematic diagram of supersaturated solid solution producedthrough quenching. FIG. 9B is a schematic diagram of a post annealingwithin the two phase region of the phase diagram which initiates thecoalescence of the second phase into ordered nano-precipitates. FIG. 9Cis a schematic diagram of a coherent nano-particle which has beenformed. FIG. 9D is a PbTe—CdTe phase diagram. FIG. 9E is a graph ofPbTe—CdTe x % reaction and post annealing profile for 2≦x≦9. FIGS. 9Fand 9G are TEM images of PbS—PbTe6%, and FIGS. 9H and 9I are TEM imagesof PbTe—CdTe9%.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The bulk materials containing nanometer-sized inclusions provideenhanced thermoelectric properties. The thermoelectric figure of meritis improved by reducing the thermal conductivity while maintaining orincreasing the electrical conductivity and the Seebeck coefficient.Coherent nanometer sized inclusions in a matrix can serve as sites forscattering of phonons that subsequently lower the thermal conductivity.General methods for preparation of these materials have been developed.

The thermoelectric heat to electricity converters will play a key rolein future energy conservation, management, and utilization.Thermoelectric coolers also play an important role in electronics andother industries. More efficient thermoelectric materials need to beidentified in order to extend their use in power generation and coolingapplications.

As previously noted, the measure used to determine the quality of athermoelectric material is the dimensionless figure of merit ZT, whereZT=(σS²/K)T, where σ is the electrical conductivity, S the Seebeckcoefficient or the absolute thermopower, T is the temperature and κ isthe thermal conductivity. The quantity σ·S² is called the power factor.The goal is then to simultaneously improve the thermopower, electricalconductivity (i.e. the power factor), and reduce the thermalconductivity thereby raising ZT. The aforementioned properties areintimately related.

PbTe and Si/Ge alloys are the current thermoelectric materials used forpower generation. These compounds once doped possess a maximum ZT ofapproximately 0.8 at 600 K and 1200 K respectively. By lowering thethermal conductivity of these materials the ZT can be improved withoutsacrificing the properties already known. Currently Bi₂Te₃ and itsalloys with Bi₂Se₃ and Sb₂Te₃, along with alloys of Bi and Sb, areconsidered the state of the art in terms of thermoelectric coolingmaterials. These materials have been modified in many ways chemically inorder to optimize their performance, however significant improvementscan be made to enhance the properties of the currently usedthermoelectric materials.

Increasing the efficiency of a thermoelectric material usually involvesraising the scattering rate of phonons while at the same timemaintaining high carrier mobility. In this respect, it has beendemonstrated that thin-film superlattice materials have enhanced ZT thatcan be explained by the decrease in the thermal conductivity. Asupperlattice structure creates de facto a complex arrangement ofstructural interfaces which in effect raises the thermal resistance ofpropagating phonons. On the other hand, lattice-matching and coherenceof the interfaces ensures undisturbed electron flow thus maintaining ahigh mobility. This decoupling of electrical and lattice thermalconductivity is necessary to reduce the total thermal conductivitywithout sacrificing the electrical conductivity.

The drawbacks of superlattice thin films are that they are expensive toprepare, difficult to grow, and will not easily support a largetemperature difference across the material. It is thus desirable toincorporate inclusions on the nanometer length scale into a bulkmaterial that is low cost, easy to manufacture, and can support atemperature gradient easily.

In the present invention, three methods have been employed in theproduction of the desired nanocomposite material for thermoelectricmaterial fabrication. Each of these methods are discussed in detail inthe following sections along with an example, Transmission ElectronMicroscope (TEM) images, and a table of materials systems that can beproduced from each general method. The first, spinodal decomposition,has been used to create a material with compositional fluctuations onthe nanometer length scale. The other two methods, matrix encapsulationand nucleation and growth, have shown the ability to produce inclusionsof various materials inside a host matrix.

Phonon mean free paths, ι_(ph), in semiconducting crystals are in therange 1≦ι_(ph)≦100 nm with a tendency to decrease with increasingtemperatures. Realization of nano-composite thermoelectric materialsoffer a way of introducing nano-meter sized scatterers that can greatlysuppress the lattice thermal conductivity through phonon scattering. Theexistence of a wide particle size distribution offers the possibility ofscattering a wider range of the phononic spectrum.

Experimental confirmation of the above at room temperature and belowcomes from the PbTe—PbS 16% at. system as the plot of the latticethermal conductivity shows in FIG. 1, a >40% reduction of the latticethermal conductivity is observed in the case of the nano-precipitatespecimen with respect to the perfect mixture of the same composition atroom temperature.

It has been suggested that band gap or electron energy statesengineering offer an alternative route to further enhancing the powerfactor of thermoelectric materials. Essentially the idea consists in themixing of parabolic bands (bulk semiconductors) with reduceddimensionality structures (e.g. nano-dots exhibit a comb-like density ofstates) to produce a ripple effect on the resulting density of states ofthe composite.

Experimental confirmation of enhanced power factors come from thefollowing systems shown in Table 1:

TABLE 1 Power factor Temperature Sample composition (μW/cmK²) (K)PbTe—PbS 16%: PbI₂ 28 400 0.05% PbTe—CdTe 5%: PbI₂ 30 300 0.05%PbTe—CdTe 9% 26 300 PbTe—Sb 4% 20 300 PbTe—InSb 2% 21 300 PbTe—Pb(0.5%)—Sb (2%) 28 300 PbTe—Pb (2%)—Sb 3%) 19 300

Method 1: Spinodal Decomposition

Spinodal decomposition refers to the way a stable single-phase mixtureof two phases can be made unstable. Thermodynamically, the necessarycondition for the stability or metastability of a heterogeneous phase isthat the chemical potential of a component must increase with increasingdensity of that component. For two components this reduces to

${{\quad\frac{\partial^{2}G}{\partial^{2}X}}_{T,P} > 0},$

where X is the concentration. If this condition is not met, the mixtureis unstable with respect to continuous compositional variations and thelimit of this metastability is called the spinodal defined as

${\frac{\partial^{2}G}{\partial^{2}X} = 0},$

where X is the concentration. Spinodal fluctuations do not involve anycrystalline transformation, since both components of the mixed phasesystem are sharing the same lattice, but involves a spatial modulationof the local composition at the nanoscale. This spatial modulation wasexploited to create coherently embedded nano-particles of a phase into athermoelectric matrix and thus create nanostructured thermoelectricmaterials on a large reaction scale.

Consider a phase diagram with a miscibility gap, i.e. an area within thecoexistence curve of an isobaric or an isothermal phase diagram wherethere are at least two phases coexisting (see FIG. 2A). If a mixture ofphases A and B and of composition X_(o) is solution treated at a hightemperature T₁ and then quenched at to a lower temperature T₂ thecomposition instantly will be the same everywhere (ideal solid solution)and hence the system's free energy will be G_(o) on the G(X) curve.However, infinitesimal compositional fluctuations cause the system tolocally produce A-rich and B-rich regions. The system now has becomeunstable since the total free energy has decreased. In time, the systemdecomposes until the equilibrium compositions X₁ and X₂ are reachedthroughout the system (compare FIG. 2A and FIG. 2B).

There are two major advantages in the application of the spinodaldecomposition process in order to produce thermoelectric nanocomposites;(a) thermodynamic principles define the spatial modulation wavelength λto be in the range nm which is a very desirable phonon-scattering lengthscale and (b) the nano-structure is thermodynamically stable. Therefore,spinodally decomposed thermoelectric materials are naturally producedbulk nanocomposites which can be perpetually stable when used within aspecified temperature region defined by the phase diagram.

The aforementioned procedure was applied extensively in the PbTe—PbSsystem where PbTe serves as the matrix.

Example PbTe—PbS x % Preparation Example

Spinodal decomposition in the two components system PbTe—PbS x % occursfor ˜4≦x≦96% for temperatures roughly below 700° C. (see accompanyingphase diagram FIG. 3A). The high purity starting materials are mixed inaqua regia cleaned fused silica tubes and fired according to thereaction profile shown in FIG. 3B.

The TEM images of spinodally decomposed system PbTe—PbS 16% are shown inFIGS. 3C, 3D.

The following Table 2 show systems that can be produced to exist in ananostructured state via the Spinodal Decomposition mechanism. Thelisting is a set of materials composed of component A and B in aA_(1-X)B_(x) stoichiometry (0<x<1).

TABLE 2 Spinodal Decomposition A-B PbTe—PbS AgSbTe₂—SnTe PbS—PbTeSnTe/SnSe SnTe/PbS SnTe/PbSe SnTe/SnSe SnSe/PbS SnSe/PbSe SnSe/PbTeAgSbSe₂—SnTe AgSbTe₂—SnSe AgSbSe₂—PbTe AgSbS₂—PbTe SnTe—REPn (RE = rareearth element, Pn = P, As, Sb, Bi) PbTe—REPn (RE = rare earth element,Pn = P, As, Sb, Bi) PbSe—REPn (RE = rare earth element, Pn = P, As, Sb,Bi) SnSe—REPn (RE = rare earth element, Pn = P, As, Sb, Bi)

Method 2: Matrix Encapsulation

These systems, as described by a phase diagram, should have a solidsolution in the composition range of approximately 0.1-15% of the minorphase. However, it has been observed that, when quenched from a melt,these systems exhibit inclusions on the nanometer scale of the minorphase material. This phenomenon can be extended to other systems ofthermoelectric interest where the matrix is a good thermoelectric andthe minor phase is a material that is nonreactive, has a lower meltingpoint, and is soluble with the matrix in the liquid state. The minorphase may also be a mixture of two or more of these non-reactivematerials which mayor may not form a compound themselves. Thesematerials must be quenched quickly through the melting point of thematrix in order to freeze the minor phase. After quenching the samplesmust be post annealed to improve crystallinity and thermoelectricproperties.

Example

This method has been applied to PbTe—Sb, PbTe—Bi, PbTe—InSb andPbTe—Pb—Sb showing promise in each of the cases.

PbTe—Sb 4% Preparation Example

Lead telluride and antimony were combined in the appropriate molar ratioand sealed in an evacuated fused silica tube and heated according to theprofile shown in FIG. 4B. The bright field and dark field images areshown in FIGS. 4C to 4E. The TEM images of encapsulated nanoparticlesare shown in FIGS. 4F to 4K. FIG. 4L shows the lattice thermalconductivity.

Table 3 shows systems for Matrix Encapsulation listing the matrix andprecipitate.

Matrix Encapsulation Using Two or More Types of Nanophase Particles:

It is possible to produce samples via the matrix encapsulation methodwhich have multiple nanoscale inclusions (two or more from those listedin Table 3).

These inclusions may be used to combine the favorable properties of eachto produce a superior thermoelectric material. The additional phasesmust also be soluble with the matrix in the liquid state, mayor may notbe reactive with the matrix, and mayor may not form a compound betweeneach other. This method has been applied to PbTe with inclusions of bothSb and Pb with interesting behavior in terms of both the reduction ofthe thermal conductivity, and modification of the behavior of theelectrical transport as well. The ratio of Pb to Sb can modify theconductivity such that a higher electrical conductivity may bemaintained through the desired temperature range. The mass fluctuationsassociated with the additional phase reduce the thermal conductivity asseen in the previously discussed examples.

PbTe—Pb—Sb Preparation Example

Pb, Sb, and Te were sealed in an evacuated fused silica tube and heatedto the molten state. The tube was then removed from the high temperaturefurnace for rapid cooling of the melt. This procedure is similar tothose discussed above, however multiple nanoprecipitate inclusion phasesare used rather than a single component inclusion. Many differentpossible inclusion combinations are possible and one example, thePbTe—Pb—Sb case, is given below.

SEM micrographs (FIGS. 5A and 5B), Powder X-ray diffraction (FIGS. 6Aand 6B), TEM micrographs (FIGS. 7A and 7B), and experimental powerfactors and thermal conductivity values (FIG. 8). These systemsrepresent an interesting set of materials in which the transportproperties can be tuned by several variables such as totalconcentration, ratio of various inclusion phases, and the properties ofthe inclusions themselves. Optimization is still underway and ZT valuesof over 1 have been obtained in the as-prepared systems.

TABLE 3 Matrix Encapsulation A(matrix)-B(precipitate)Pb(Te,Se,S)—Sb,Bi,As) Pb(Te,Se,S—(InSb,GaSb) Pb(Te,Se,S)—YbPb(Te,Se,S)—(InAs,GaAs) Pb(Te,Se,S)—Eu Pb(Te,Se,S)—In Pb(Te,Se,S)—GaPb(Te,Se,S)—Al Pb(Te,Se,S)—Zn Pb(Te,Se,S)—Cd Pb(Te,Se,S)—SnPb(Te,Se,S)—T1InQ2* Pb(Te,Se,S)—ZnxSby Pb(Te,Se,S)—AgYb Pb(Te,Se,S)—CdPdPb(Te,Ae,S)—REPb3 (RE = rare earth element,Y) Pb(Te,Se,S)—M (M = Ge, Sn,Pb) Pb(Te,Se,S)—Ag4Eu Pb(Te,Se,S)—AgEu Pb(Te,Se,S)—AgCePb(Te,Se,S)—Mg2Cu Pb(Te,Se,S)—Cu2La Pb(Te,Se,S)—Cu6Eu Pb(Te,Se,S)Eu3Pd2Pb(Te,Se,S)—Mg2Eu Pb(Te,Se,S)—PdTe2 Pb(Te,Se,S)—Mg2Sn Pb(Te,Se,S)—MgSmPb(Te,Se,S)—MgPr Pb(Te,Se,S)—Mg2Pb Pb(Te,Se,S)—Ca84Ni16Pb(Te,Se,S)—RE2Pb (RE = rare earth element) Pb(Te,Se,S)—Mg2PbPb(Te,Se,S)—REPb (RE = rare earth element) *Q = S,Se,Te

Method 3: Nucleation and Growth Mechanism:

The method of nucleation and growth of nanoparticles within the matrixof a thermoelectric material consists of three distinct thermaltreatments that depend crucially on the phase diagram of the composite:

a) The starting materials (mixed in appropriate stoichiometry) areheated from the two-phase region to the single-phase region of the phasediagram to dissolve all precipitates. The mixture is held there forseveral hours to ensure complete homogeneity;

b) The melt or solid solution is quenched to room temperature usingdifferent methods: air quenching, water quenching, ice water quenching.This freezes the high temperature homogenous phase into a supersaturatedsolid solution; and

c) Depending on the kinetics of the specific system the specimen is postannealed at an elevated temperature within the two-phase region of thephase diagram and is held there for several hours to allow thenanoprecipitates to form and grow. Annealing time and temperature isproportional to the size growth of the precipitates. Therefore, the sizeof the nanoprecipitates can be controlled through careful selection ofannealing time and temperature.

The following schematic shows in FIGS. 9A, 9B and 9C roughly how thenano-precipitation of the second phase is taking place.

As a general rule this kind of nanostructured thermoelectric materialsshould meet two conditions: (1) The two phases should contain elementsthat enter a solid solution phase at a specific temperature and separateinto a mixture at another lower temperature. (2) The phase thatprecipitates out must create a coherent or at best semi-coherentprecipitate. Coherency is important since it ensures bonding with thelattice of the matrix and hence the precipitate does not act as a strongscatterer to the electrons.

The above procedure has been extensively applied to the PbTe—CdTe systemwith excellent results.

Example PbTe—CdTe x % Preparation Example

Stoichiometric quantities of Pb, Te and Cd are weighed targeting x %values in the range 2≦x≦9. The starting materials are placed in graphitecrucibles, which are subsequently sealed under high vacuum in fusedsilica tubes and fired according to the reaction profile shown below(FIG. 9E). The reaction profile is decided based on the phase diagram ofthe PbTe—CdTe system (FIG. 9D). FIGS. 9F and 9G show the TEM images forprecipitation and growth of PbS—PbTe 6%. FIGS. 9H and 9I show the systemPbTe—CdTe 9%.

The following Table 4 shows systems for nucleation and growth listingthe matrix and precipitate.

TABLE 4 Nucleation and Growth A(matrix)-B(precipitate) PbSe—Sb₂Se₃PbSe—SnSe₂ PbSe—Zn PbTe—Hg_(1−x)Cd_(x)Te (0 < x < 1) PbTe—ZnTePbTe—Sb₂Se₃ PbTe—Zn PbTe—In₂Se₃ PbTe—In₂Te₃ PbTe—Ga₂Te₃ PbTe—AgInTe₂PbTe—CuInTe₂ PbTe—CuInSe₂ PbTe—CuInTe₂ PbSe—Hg_(1−x)Cd_(x)Q (0 < x < 1,Q = S, Se, Te) PbSe—ZnTe PbSe—In₂Se₃ PbSe—In₂Te₃ PbSe—Ga₂Te₃PbSe—AgInSe₂ PbSe—CuInTe₂ PbSe—CuInSe₂ PbSe—CuInTe₂

It is intended that the foregoing description be only illustrative ofthe present invention and that the present invention be limited only bythe hereinafter appended claims.

1. A thermoelectric composition comprising: a matrix comprising a firstchalcogenide; and nanoscale inclusions in the matrix, the nanoscaleinclusions being coherent or semi-coherent with the matrix, thenanoscale inclusions having a different composition than the firstchalcogenide, so that the nanoscale inclusions decrease the thermalconductivity of the composition by scattering phonons in the compositionwhile substantially maintaining or increasing electrical conductivityand Seebeck coefficient of the composition.
 2. The thermoelectriccomposition of claim 1, wherein the nanoscale inclusions are coherentwith the matrix.
 3. The thermoelectric composition of claim 1, whereinthe nanoscale inclusions have a first melting point, the matrix has asecond melting point, and the first melting point is lower than thesecond melting point.
 4. The thermoelectric composition of claim 1,wherein the nanoscale inclusions have a first melting point, the matrixhas a second melting point, and the second melting point is lower thanthe first melting point.
 5. The thermoelectric composition of claim 1,wherein the nanoscale inclusions have a first melting point, the matrixhas a second melting point, and the first melting point is differentthan the second melting point.
 6. The thermoelectric composition ofclaim 1, wherein at least a portion of the nanoscale inclusions are auniform dispersion of nanoparticles.
 7. The thermoelectric compositionof claim 1, wherein about 0.1 to 15% of the composition comprises thenanoscale inclusions.
 8. The thermoelectric composition of claim 1,wherein at least a portion of the nanoscale inclusions comprise amaterial that is nonreactive, has a lower melting point, and is solublewith the matrix in a liquid state.
 9. The thermoelectric composition ofclaim 1, wherein the first chalcogenide comprises a chalcogen selectedfrom the group consisting of tellurium, sulfur and selenium.
 10. Thethermoelectric composition of claim 1, wherein at least a portion of thenanoscale inclusions have a size between about 1 and 200 nanometers. 11.The thermoelectric composition of claim 1, wherein the nanoscaleinclusions comprise multiple types of inclusions, each type having adifferent chemistry.
 12. The thermoelectric composition of claim 1,wherein the composition has lattice thermal conductivity which is morethan 40% reduced as compared to lattice thermal conductivity of thematrix.
 13. The thermoelectric composition of claim 1, wherein thematrix comprises PbTe.
 14. The thermoelectric composition of claim 13,wherein the nanoscale inclusions comprise PbS.
 15. The thermoelectriccomposition of claim 13, wherein the nanoscale inclusions comprise atleast one element selected from the group consisting of antimony,bismuth, and arsenic.
 16. The thermoelectric composition of claim 13,wherein the nanoscale inclusions comprise lead and antimony.
 17. Thethermoelectric composition of claim 13, wherein the nanoscale inclusionscomprise Cd_(1-x)Hg_(x)Te and 0<x<1.
 18. The thermoelectric compositionof claim 1, wherein the matrix comprises PbS and the nanoscaleinclusions comprise PbTe.
 19. The thermoelectric composition of claim 1,wherein the nanoscale inclusions comprise a metal.
 20. Thethermoelectric composition of claim 1, wherein the nanoscale inclusionscomprise a semiconductor.
 21. The thermoelectric composition of claim 1,wherein the nanoscale inclusions comprise a second chalcogenidedifferent from the first chalcogenide, the second chalcogenidecomprising a chalcogen selected from the group consisting of tellurium,sulfur and selenium.
 22. The thermoelectric composition of claim 1,wherein the matrix comprises PbQ, and the Q component comprises at leastone element selected from the group consisting of: tellurium, selenium,and sulfur.
 23. The thermoelectric composition of claim 1, wherein thematrix comprises SnQ, and the Q component comprises at least one elementselected from the group consisting of: tellurium and selenium.
 24. Thethermoelectric composition of claim 1, wherein the nanoscale inclusionsdo not act as a strong scatterer to electrons.
 25. The thermoelectriccomposition of claim 1, wherein at least a portion of the inclusions inthe matrix are thermally stable to a temperature higher than 650 K. 26.The thermoelectric composition of claim 1, wherein the compositioncomprises a bulk composition.
 27. A thermoelectric compositioncomprising: a matrix comprising a first chalcogenide; and nanoscaleinclusions in the matrix, the nanoscale inclusions have a first meltingpoint, the matrix has a second melting point, the first melting point islower than the second melting point, the nanoscale inclusions having adifferent composition than the first chalcogenide, so that the nanoscaleinclusions decrease the thermal conductivity of the composition byscattering phonons in the composition while substantially maintaining orincreasing electrical conductivity and Seebeck coefficient of thecomposition.
 28. A thermoelectric composition comprising: a matrixcomprising a first chalcogenide; and nanoscale inclusions in the matrix,the nanoscale inclusions comprise a second chalcogenide different fromthe first chalcogenide, the second chalcogenide comprising a chalcogenselected from the group consisting of tellurium, sulfur and selenium, sothat the nano scale inclusions decrease the thermal conductivity of thecomposition by scattering phonons in the composition while substantiallymaintaining or increasing electrical conductivity and Seebeckcoefficient of the composition.